Return flattened array from each element in a nested array mongo with aggregation query

Question

I have a collection setup with documents that look like :

     {
        "_id" : ObjectId("5c786d9486c1140b1452d777"),
        "code" : "TEST-123",
        "owner" : "John",
        "cars" : [ 
            {
                "carPlate" : "QPZ-756",
                "carColor" : "blue"
            }, 
            {
                "carPlate" : "REF-473",
                "carColor" : "red"
            }
        ],
     }

I'm looking for an mongo aggregate query that grabs each carPlate and outputs the following for every document in the collection

        {
           "carPlate" : "QPZ-756",
           "owner" : "John",
           "code" : "TEST-123",
        },
        {
           "carPlate" : "REF-473",
           "owner" : "John",
           "code" : "TEST-123",
        },

I had a look at the $map operator, would this be a good place to start?

s7vr · Accepted Answer · 2019-03-01 01:30:10Z

1

I would use $unwind to flatten the array followed by $mergeObjects to combine keys along with $replaceRoot to promote the merge documents to the top.

Something like

db.colname.aggregate([
    {$unwind:"$cars"},
    {$replaceRoot:{newRoot:{$mergeObjects:[{owner:"$owner"}, "$cars"]}}}
])

answered Mar 1, 2019 at 1:30

s7vr

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